1. Field of the Invention
This invention relates to novel lubricant additives and the preparation thereof. More particularly, it relates to improvement in the viscosity index of mineral oil of lubricating viscosity by addition thereto of a certain class of saturated ethylene copolymers which provide said mineral oil with excellent low temperature viscometric properties relative to other ethylene copolymeric viscosity-index improvers.
2. Description of the Prior Art
As is well known to those skilled in the art, lubricating oils may be evaluated by many criteria each of which relates to the proposed use of the oil. One of the more important of these criteria is the viscosity index.
It is known that the viscosity index of lubricating oils can be usefully modified by the addition of oil-soluble polymeric viscosity index (V.I.) improvers such as polyisobutylenes and ethylene copolymers including ethylene-higher alpha-olefin copolymers and terpolymers; however, such an addition can introduce chemical instability.
Recently, ethylene-propylene copolymers have become widely used as viscosity improvers in lubricating oils because of the low treat levels and improved viscometric properties. However, the market requires different molecular weight grades, which have different degrees of thickening effect - usually called Thickening Efficiencies or T.E. - so as to be operable with different viscosity mineral oils. Although the preparation of each of such copolymer grades can be by direct synthesis, the different molecular weight grades can be produced by degradation of an ethylene-propylene copolymer so as to produce lower molecular weight versions. It is advantageous to use the degradation approach since it is more economical to make a large run of copolymer in a large scale polymer plant, and then to use the copolymeric product of said run as a base material which is then broken down into lower molecular weight grades in order to meet requirements of the viscosity index improver market. There are various means to achieve such degradation including: heating an amorphous rubbery ethylene-propylene copolymer for 3 to 30 minutes at from 260.degree. to 420.degree. C. with the exclusion of air; extruding and heating an ethylene copolymer first at 150.degree.-280.degree.,C. until molten and then at a temperature of 300.degree.-500.degree. C. with the exclusion of air; and, by oxidative and mechanical degradation, as by mastication of ethylene-propylene copolymers in an oxygen-containing environment which copolymers are preferably free of other monomers such as dienes (see U.S. Pat. No. 3,769,216).
It is known that ethylene terpolymers which contain dienes, e.g. 5-vinyl-2-norbornene or ethylene tetrapolymer which contain both 1,4-hexadiene and 2,5-norbornadiene are not suitable for mechanical degradation as by mastication in the presence of air or oxygen whereby oxidation occurs since this degradation technique produces excessive amounts of polymeric gel particles which are oil insoluble. This problem has been overcome by use of an ethylene copolymer containing also a C.sub.3 to C.sub.18 higher alpha-olefin and from 1 to 25 wt. % of an alkyl norbornene having from 8 to 28 carbons (see German DOS 28 15 427 published October 19, 1978).
The patent literature is replete with many publications dealing with ethylene tri- and tetrapolymers containing one or more types of dienes introduced for a variety of reasons including a means to introduce branchiness into the ethylene polymer and to provide a means for crosslinking said polymer through introduction of a crosslinking agent reactive with a portion of said diene, e.g. see U.K. Patent Specification 1,195,625. Those patents, however, dealing with ethylene tri- and tetrapolymers added to a mineral oil as a viscosity index modifying additive are of limited number and are best illustrated by the specification of U.S. Pat. No. 3,790,480. This specification teaches of ethylene ter- and tetrapolymers involving ethylene, a C.sub.3 to C.sub.18 higher alpha-olefin, for example, propylene, and two classes of dienes based upon the relative polymerizabilities of each of the double bonds. In one class of dienes (as represented by 1,4-hexadiene) only one of the double bonds is readily polymerizable whereas in the other class (as represented by 2,5-norbornadiene) both double bonds are readily polymerizable. It is taught therein that an ethylene polymeric viscosity index additive for mineral oils is superior when and if it is an ethylene tetrapolymer containing both classes of dienes rather than the prior art ethylene terpolymer containing the class of dienes having only one readily polymerizable double bond. Apparently, this superiority obtains because the introduction of the second diene comonomer with two readily polymerizable double bonds into the terpolymer composition provides a significant increase in bulk polymer viscosity with only a minor increase of the inherent viscosity (see col. 8, lines 23-30) and without degradation of the property of the terpolymer to provide viscosity index improvement to mineral oils. The only discussion of the low temperature viscometrics of mineral oil solutions containing this type of ethylene terpolymer is set forth in col. 10, lines 1-3 wherein it is stated that they gave satisfactory results for 10W30 multigrade lubricating oils.
U.S. Pat. Nos. 3,499,741 and 3,681,302 discuss ethylene terpolymers containing a minor amount of a nonconjugated diene which are usefully thermally cracked to provide a pour point depressant additive for "fuel oils, diesel oils, middle distillates and other viscous hydrocarbon oils" (see col. 1, first paragraph of specifications). The aforesaid terpolymer is also useful as an additive for gasoline to modify induction system deposits (see col. 1, lines 23-25 of '741). The 9 specified non-cojugated dienes include 2,5-norbornadiene, i.e. bicyclo [2,2,1] hepta-2,5-diene] (see col. 2, line 36 of '302 also col. 2, lines 29-30 of '741) although the invention is specifically illustrated only with thermally cracked, unsaturated ethylene-propylene-hexadiene-1,5 and ethylene-propylene-cyclopentadiene terpolymers.
Unsaturation in a polymeric viscosity index improving additive is generally undesirable since the unsaturated moiety, i.e. the double or triple bond of the polymer represents a means by which the desired properties of the additive can be degraded during the lubricating of the engine, e.g. as by cleavage of the high molecular polymer into fragments no longer possessing viscosity index modifying properties or by crosslinking of said polymer to a molecular weight which is no longer mineral oil soluble provoking its precipitation onto interior engine surfaces as a varnish and/or contributing to engine lubricating oil sludge.
It is an object of this invention to provide a saturated ethylene copolymeric viscosity index improver for mineral oils of lubricating viscosity of excellent low temperature viscometrics, preferably one that can be readily oxidatively degraded to a lower molecular weight in order to achieve maximum operational properties.